| dc.description.abstract | The fate of SOC upon entering a river has remained largely unknown. Whereas conventional global carbon cycle models often regard river transport as a passive channel, several studies have emphasized that OC oxidation may play an important role during transport. To understand more about the properties of SOC that is discharged into the ocean and is available for carbon sequestration, it is vital to determine its provenance. For tracing SOC through a river catchment, the relative distribution of SOC-specific membrane lipids (brGDGTs) was previously employed. However, when in situ aquatic production of these brGDGTs occurs in the water column (i.e. during transport), it has proven to potentially supress the original soil-derived brGDGT signal. The recently described 6-methyl brGDGT isomers were primarily associated with primary production in a Siberian river system and may aid in identifying primary production on basis of brGDGT distribution. This study aimed to segregate these two brGDGT signals to improve the determination of the provenance of SOC that is finally discharged. Secondary goals were to evaluate the effect of increased precipitation on SOC transport and its provenance, and produce a dataset showing regional variation of bulk soil properties and brGDGT distributions throughout a catchment.
In this study, I have investigated the faith of SOC during transport in the monsoon driven Godavari River system. The geographically sharply divided soil types (relatively acidic vs relatively alkaline in the eastern and western catchment area, respectively) and bimodular precipitation regime and consequent discharge mode (baseflow characterized by high primary production versus soil erosion and transport) in the catchment area made it a perfect case study to segregate the brGDGT distributions of soil and aquatic origin.
In February (dry season) and July-August (monsoon season) 2015, soil (n = 46, dry season), SPM (n = 20 and n = 49 from dry and monsoon season, respectively) and river sediment (n = 74, 37 from each season) samples were collected in the Godavari catchment area. Physical properties were determined and reported for Godavari soils (SOC, soil pH), Godavari River water at SPM and sediment sampling locations (δ18O, δ2H, temperature, pH, EC, sediment loading) and Godavari River sediment (TOC, pH, surface area). All samples (soil, SPM and river sediment) were freeze dried, homogenized, extracted, and then analysed for GDGTs, using an UHPLC – APCI – MS system, following method described by Hopmans et al. (2016).
Results showed that SOC transported is severely limited during the dry season: sediment loading is very low and the relative brGDGT distribution in SPM differed substantially from that of soil samples, throughout the catchment. While all samples were generally dominated by the tetramethylated brGDGT Ia, the dry season SPM samples were characterized by an increase in abundance of 6-Me isomers of penta- and hexamethylated brGDGTs.
During the monsoon season, the sediment loading of the Godavari River discharge increased exponentially. SOC erosion and transported occurred preferentially at the eastern side of the catchment area, as inferred from its sediment loading and the strong match between SPM and soil brGDGT distribution patterns, which were both characterized by a high abundance of brGDGT Ia. However, the western side of the catchment showed a brGDGT distribution pattern characterized by the 6-Me penta-and hexamethylated brGDGTs, similar as during the dry season, and seemed not to contribute to the final SOC discharge in the Godavari Delta.
Although brGDGTs can adequately be employed to trace SOC through a catchment, the processes affecting their fate during transport need further investigation before their distribution can be effectively and accurately applied in paleo-environmental reconstructions, as without a reference on the provenance of SOC it remains impossible to segregate the soil-derived brGDGT signal from transformations during transport. | |
